[automatic control system of liquid chillers]

ABSTRACT

An automatic control system of the invention comprises a plurality of liquid chillers, a principal controller, and a plurality of secondary controllers respectively connected to the liquid chillers and the principal controller. The automatic control system stores a British Thermal Unit (BTU) table that includes a predetermined high load limit parameter and a predetermined low load limit parameter varying according to the number of liquid chillers in operation. The principal controller compares an actual load of the currently operating liquid chillers with the high and/or low load limit parameter to determine an adequate number of liquid chillers that have to be operated. By evaluating the number of liquid chillers currently operating, the principal controller then determines whether liquid chillers have to be switched on or switched off through the secondary controllers.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of Taiwanapplication serial no. 91123555, filed on Oct. 14, 2002.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to an automatic control system ofliquid chillers and, more particularly, to an automatic control systemthat controls the liquid chillers based upon a BTU table.

[0004] 2. Description of the Related Art

[0005] Liquid chilling machines (also called “liquid chillers”) areconventionally used to process in low temperature conditions. Moreparticularly, when the low temperature conditions have to be maintainedwith a relatively high precision, an automatic control system of theliquid chillers is needed to prevent undesirable temperaturefluctuations due to malfunctions of some of the liquid chillers.

[0006] Referring to FIG. 1, a simplified schematic view illustrates anautomatic control system of liquid chillers known in the prior art. Theknown automatic control system 10 comprises a primary loop 12 and asecondary loop 14 that are connected to each other via a de-couplingpipe 18. Chilled liquid outputted by the liquid chillers 102 travelsthrough a chilled liquid pipe 106 to a plurality of control valves 112and secondary pumps 114, and is delivered through a chilled liquidoutlet. Through a liquid inlet, the chilled liquid after having beenused returns through the chilled liquid pipe 106 to the primary pumps104. However, a part of this used liquid also flows through the pipe 108to the control valves 112, which causes an increase in temperature ofthe chilled liquid at the chilled liquid outlet.

[0007] Along the chilled liquid pipe 106 are mounted a plurality oftemperature sensors 110 to sense the temperature of the liquid flowingthrough along the pipe 106. Any abnormal temperature variation therebyis detected, which enables to take adequate decisions such as increasingthe load of currently operating liquid chillers or, if necessary,activating additional liquid chillers.

[0008] The conventional operation of the control system as describedabove is based upon the detection of the temperature of the circulatingliquid. The mount of several temperature sensors is therefore needed,which increases the equipment cost.

SUMMARY OF INVENTION

[0009] An aspect of the invention is therefore to provide an automaticcontrol system of liquid chillers that determines the on-switch oroff-switch of liquid chillers according to the actual load of currentlyoperating liquid chillers compared against an acceptable load limit ofthe liquid chillers.

[0010] To accomplish the above and other objectives, an automaticcontrol system of the invention comprises a plurality of liquidchillers, a principal controller, and a plurality of secondarycontrollers respectively connected to the liquid chillers and theprincipal controller. The automatic control system stores a BritishThermal Unit (BTU) table that includes a predetermined high load limitparameter and a predetermined low load limit parameter varying accordingto the number of liquid chillers in operation. The principal controllercompares an actual load of the currently operating liquid chillers withthe high or low load limit parameter to determine an adequate number ofliquid chillers that have to be operated. By evaluating the number ofliquid chillers currently operating, the principal controller thendetermines whether liquid chillers may have to be switched on orswitched off. To achieve the above purpose, a plurality of secondarycontrollers are respectively connected between the liquid chillers andthe principal controller. To switch on or switch off liquid chillers,the principal controller sends a corresponding command signal to thesecondary controllers that accordingly switch on or switch off theproper liquid chillers and equipment associated with the liquidchillers.

[0011] According to an embodiment of the invention, the principalcontroller includes a plurality of control units among which isalternately switched a control right. Thereby, an adequate control ofthe liquid chillers is maintained even if one control unit has amalfunction.

[0012] According to the invention, an on-switch delay time and anoff-switch delay time are further set in the BTU table. The on-switchdelay time indicates a delay time after which the liquid chiller to beswitched on is effectively switched on. This delay time allows aconfirmation that the outputted on-switch command signal corresponds toa real need of capacity increase and not due to erroneous noise signals,which therefore prevents undesirable on-switch operations. Theoff-switch delay time indicates a delay time after which the liquidchiller to be switched off is effectively switched off, which preventserroneous off-switch operations.

[0013] The equipment associated with the liquid chillers compriseschilled liquid pumps, liquid recycle pumps, and control valves.

[0014] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

[0016]FIG. 1 is a schematic view illustrating an automatic controlsystem of liquid chillers known in the art.

[0017]FIG. 2 is a schematic view illustrating the general structure ofan automatic control system of liquid chillers according to anembodiment of the invention.

[0018]FIG. 3 is a chart schematically illustrating a BTU table used bythe automatic control system according to an embodiment of theinvention.

[0019]FIG. 4 is a flow chart illustrating the control operation of anautomatic control system according to an embodiment of the invention.

[0020]FIG. 5 is state diagram of an automatic control system accordingto an embodiment of the invention.

[0021]FIG. 6 is a flow chart illustrating the control operation of asecondary chilled liquid pump system according to an embodiment of theinvention.

DETAILED DESCRIPTION

[0022] The following detailed description of the embodiments andexamples of the present invention with reference to the accompanyingdrawings is only illustrative and not limiting. Furthermore, whereverpossible in the description, the same reference symbols will refer tosimilar elements and parts unless otherwise illustrated in the drawings.

[0023] Referring to FIG. 2, a block diagram schematically illustrates anautomatic control system of liquid chillers according to an embodimentof the invention. A major aspect of the invention is that a principalcontroller 20 operates according to the data provided by a BTU table (asillustrated in FIG. 3) to control the operations of a plurality ofliquid chilling machines 26 (also called “liquid chillers” hereafter).The principal controller 20 compares the actual load of chilled liquidproduction with a high (and/or low) load limit in the BTU table todetermine an adequate number of liquid chillers 26 to be operated.Consequently, the principal controller 20 outputs a command signal foreither switching on or switching off liquid chillers 26 in order tooptimally satisfy the actual load of chilled liquid production. If thenumber of currently operating liquid chillers is equal to the adequatenumber of liquid chillers to be operated, no liquid chillers areswitched on or switched off.

[0024] The principal controller 20 comprises a plurality of controlunits 202 that alternately possess a control right. Therefore, if amalfunction occurs to one control unit 202 currently possessing thecontrol right, the automatic control system transfers this control rightto another control unit 202. The control units 202 are, for example,programmable logic circuits. The command signal travels through amodular bus 22 to secondary controllers 24 that, accordingly,effectively switch on or switch off liquid chillers 26 and associatedequipment 28. The equipment 28 includes at least a supply pump 282, aliquid recycle pump 284, and a control valve 286 that are respectivelyconnected to the liquid chillers 26.

[0025]FIG. 3 describes an example of BTU table that is used in anautomatic control system according to an embodiment of the invention. Inthe BTU table, the first row indicates the number of liquid chillers.The second row indicates the accumulated capacity corresponding to thenumber of liquid chillers. “High load limit” indicates a load limit ofchilled liquid production over which at least one additional liquidchiller has to be switched on to allow an optimal operation of theliquid chillers. “On-switch delay time” indicates a delay time afterwhich the liquid chiller to be switched on is effectively switched on.This delay time allows the performance of a confirmation operationensuring the outputted on-switch command signal corresponds to a steadyneed of capacity increase and not due to erroneous noise signals, whichtherefore prevents undesirable on-switch operations. “Low load limit”indicates a load limit of chilled liquid production below which at leastone liquid chiller has to be switched off. “Off-switch delay time”indicates a delay time after which the liquid chiller to be switched offis effectively switched off. Similar to the function of the on-switchdelay time, this off-switch delay time allows the performance of aconfirmation operation to ensure there is a steady capacity decrease,which prevents undesirable and erroneous off-switch operations.

[0026] Reference now is made to FIG. 2 through FIG. 4 to describe theoperation of the automatic control system of liquid chillers accordingto an embodiment of the invention. First, the actual load of chilledliquid production is compared with the high (and/or low) load limit ofthe BTU table to determine the adequate number of liquid chillers 26that have to be operated. This adequate number of liquid chillers 26 iscompared with the number of currently operating liquid chillers 26 todetermine whether liquid chillers may have to be switched on or switchedoff. Being activated, the principal controller 20 checks the state ofthe automatic control system (step S102), and establishes an on-switchsequence of the liquid chillers 26 according to their service time (stepS104). Subsequently, power is supplied to the liquid chillers 26 andequipment 28 (step S106). If power is normally supplied to each of theselected liquid chillers 26, the on-switch sequence previouslyestablished is confirmed and the liquid chillers 26 are effectivelyactivated (step S108). Otherwise, step S104 is re-executed to establisha new on-switch sequence of liquid chillers.

[0027] Meanwhile, the principal controller 20 determines the adequatenumber of liquid chillers 26 to be operated according to the BTU table(step S110), and through the modular bus 22, outputs a command signal ofthe liquid chillers 26 and equipment 28 (step S112). Subsequently, theautomatic control system supplies power to the pump 282 (step 114).After having received the command signal (step S116), the secondarycontrollers 24 activate the liquid chillers 26 and the equipment 28(step S118). While the liquid chillers 26 are operating, the automaticcontrol system continuously records the operating state and the servicetime of each of the operating liquid chillers 26 (step S120). The statesof the liquid chillers 26 and equipment 28 are feedback through themodular bus 22 to the principal controller 20.

[0028] When a liquid chiller 26 malfunctions, the principal controller20 switches to a manual control mode (step S122). From the principalcontroller 20, the secondary controllers 24 are therefore manuallycontrolled (step S124) to activate one or more liquid chiller 26 andequipment 28 (step S126). As described above, the states of the liquidchillers 26 and equipment 28 are similarly sent back through the modularbus 22 to the principal controller 20 (step S128).

[0029] Referring to FIG. 5, a state diagram illustrates an input stateof the principal controller and a system control state according to anembodiment of the invention. In FIG. 5, reference numeral 30 refers tothe principal controller, reference numeral 32 designates an inputstate, and reference numeral 36 designates a system control. The statesand parameters from the input state 32, including the states of theliquid chillers and equipment, are delivered to the principal controller30 that accordingly performs various control operations as indicated inthe system control 36. According to the actual demand in chiller liquidproduction and the parameters of the BTU table, the principal controller30 controls the operation of the automatic control system.

[0030] In the input state 32, a power state 320 indicates the state ofpower supply in the system. A liquid chiller state 322 continuouslyinforms the principal controller 30 of the operating state of the liquidchillers. If a chilled liquid temperature 324 exceeds a predeterminedlimit more than three minutes, the automatic control system activates afirst set of chilled liquid towers and the liquid chillers. The inputstate 32 also includes a pump state 326, a cooling tower status 328, aBTU and power meter 330, a liquid chiller equipment status 332, anoutdoor temperature 334, a PDT 336, a control network system state 338,and a sensor 340.

[0031] The system control 36 comprises: a liquid treatment systemcontrol 360, a 5 Â° C. on-switch/off-switch system control 362, acooling tower filtration system control 364, a cooling tower control366, a secondary chilled liquid pump control 368, a chilled liquid loadmanagement system control 370, a warm/hot liquid system control 372, a 9Â° C. on-switch/off-switch system control 374, and a plumbing systemcontrol 376.

[0032] Referring to FIG. 6, a flow chart schematically illustrates acontrol process of the secondary chilled liquid pumps according to anembodiment of the invention. According to the secondary chilled liquidpump/liquid chiller unit ratio, the principal controller determines thenumber of secondary chilled liquid pumps that may have to be activatedor stopped, and accordingly delivers a command signal (step S202).Subsequently, the automatic control system supplies power to thesecondary chilled liquid pumps (step S204), and activates the secondarychilled liquid pumps. If secondary chilled liquid pumps have to bestopped, pumps that have been operating longer are first stopped (stepS206).

[0033] After having received the command signal from the principalcontroller, the control valves respectively control the activation andthe speed of the secondary chilled pumps (step S208). The operatingstates of the secondary chilled liquid pumps are feedback to theprincipal controller (step S210). Subsequently, the on-switch/off-switchcontrol system of the secondary chilled liquid pump system is activated,and the state of the on-switch/off-switch control system isautomatically checked, the result of this check being sent to theprincipal controller (step S212).

[0034] As described above, the invention therefore includes thefollowing advantages.

[0035] (1) By comparing the actual load in chilled liquid production andthe predetermined high or low load limit of the BTU table, the automaticcontrol system determines the adequate number of liquid chillers thatshould be operating. According to the actual number of liquid chillersthat are operating, the automatic control system consequently determineswhether additional liquid chillers have to be switched on or currentlyoperating liquid chillers have to be switched off.

[0036] (2) Via a load control of the chilled liquid production from theBTU table, the invention allows an optimal operation-of the liquidchillers.

[0037] (3) The principal controller of the invention includes aplurality of control units, between which a control right can bealternately switched. The control of the entire system can be thereforeensured even if some control units have malfunctions.

[0038] It should be apparent to those skilled in the art that otherstructures that are obtained from various modifications and variationsof different parts of the above-described structures of the inventionwould be possible without departing from the scope and spirit of theinvention as illustrated herein. Therefore, the above description ofembodiments and examples only illustrates specific ways of making andperforming the invention that, consequently, should cover variations andmodifications thereof, provided they fall within the inventive conceptsas defined in the following claims.

1. An automatic control system of liquid chillers, comprising: aplurality of liquid chillers for producing chilled liquid; and aprincipal controller, operating from data of a British Thermal Unit(BTU) table, wherein the principal controller compares an actual load ofchilled liquid production with a load limit of the BTU table todetermine an adequate number of liquid chillers to be operated, andafter having compared a number of currently operating liquid chillerswith the adequate number of liquid chillers to be operated, theprincipal controller accordingly outputs a command signal to command anon-switch, off-switch, or no-operation of the liquid chillers.
 2. Thesystem of claim 1, further including a secondary controller thatreceives the command signal to either switch on or switch off at leastone of the liquid chillers.
 3. The system of claim 1, wherein theprincipal controller comprises a plurality of control units that arealternately given a control right.
 4. The system of claim 3, wherein thecontrol units are programmable logic circuits.
 5. The system of claim 1,wherein the BTU table includes a high load limit; if the actual load inchilled liquid production is higher than the high load limit, additionalliquid chillers are switched on.
 6. The system of claim 1, wherein theBTU table includes a low load limit; if the actual load in chilledliquid production is lower than the low load limit, currently operatingliquid chillers are switched off.
 7. The system of claim 1, wherein theBTU table further includes an on-switch delay time that allows aconfirmation that the outputted command signal commanding an on-switchcorresponds to a steady need of capacity increase and not due toerroneous noise signals.
 8. The system of claim 1, wherein the BTU tablefurther includes an off-switch delay time that allows a confirmationthat the outputted command signal commanding an off-switch correspondsto a steady capacity decrease and not due to erroneous noise signals. 9.The system of claim 1, wherein an on-switch/off-switch time interval isautomatically set.
 10. A control method of liquid chillers, comprising:comparing an actual load of chilled liquid production with apredetermined high and low load limit of the liquid chillers todetermine an adequate number of liquid chillers to be operated;comparing the adequate number of liquid chillers to be operated with anumber of currently operating liquid chillers; if the adequate number ofliquid chillers to be operated is greater than the number of currentlyoperating liquid chillers, at least one additional liquid chiller isswitched on; if the adequate number of liquid chillers to be operated isless than the number of currently operating liquid chillers, at leastone currently operating liquid chiller is switched off; and if theadequate number of liquid chillers to be operated is equal to the numberof currently operating liquid chillers, the currently operating liquidchillers are maintained.
 11. The method of claim 10, wherein if theadequate number of liquid chillers to be operated is greater than thenumber of currently operating liquid chillers, an additional liquidchiller is switched on after an on-switch delay time has elapsed, theon-switch delay time being used to confirm whether there is a steadyneed of capacity increase.
 12. The method of claim 10, wherein if theadequate number of liquid chillers to be operated is less than thenumber of currently operating liquid chillers, a currently operatingliquid chiller is switched off after an off-switch delay time haselapsed, the off-switch delay time being used to confirm whether thereis a steady capacity decrease.